Cultivating human embryos is a delicate issue because they have the potential to develop into viable pregnancies if implanted into wombs.1 The global scientific community has consequently operated under a general rule to limit in vitro research on human embryos to the first 14 days of development, a boundary that when first suggested in 1979 was far beyond technological capabilities to break.2 

However, that is no longer the case, and in 2021, the International Society for Stem Cell Research (ISSCR) removed the rule from its guidelines and encouraged case-by-case review.3 Yet in many countries, the rule remains enshrined in law. We asked two experts in the field whether it’s time to leave it behind.

          Image of Naomi Moris
Naomi Moris is a developmental biologist at the Francis Crick Institute. She works with 3D stem cell structures that model human embryos.

Naomi Moris

I think that we definitely need to reconsider the rule. The science is pushing up against it from multiple directions: We’re getting better and better at culturing embryos in a lab environment, and we are developing these embryo-like models, such as blastoids, that are really challenging what the word embryo means. The rule is unclear when it comes to them. The ISSCR suggestion of operating on a case-by-case basis allows us to push the boundaries and show the public the benefit of the research before having a wider discussion about going further. It’s probably the most workable solution given how fast the science is moving.

          Image of Kirstin Matthews
Kirstin Matthews is a policy scholar in biomedical research at Rice University’s Baker Institute for Public Policy; she has a background in molecular biology.

Kirstin Matthews

It would be fine to withdraw the 14-day rule if we replace it with another guardrail that communicates that we, as a profession, have guidelines and that we consider embryos to be special entities. We did an assessment4 where I brought in scholars who were hesitant about relaxing the rule, and a lot of what they wanted was to ensure that the science was thoughtful and respected public beliefs, whereas the people interested in removing the restriction focused on what knowledge could be gained. There is probably a compromise where everyone’s slightly unhappy, but I think that’s where we need to go.

These interviews have been edited for length and clarity.


  1. Amato P, et al. Fertil Steril. 2020;113:270-294.
  2. Hyum I, et al. Nat. 2016;533:169-171.
  3. ISSCR Guidelines, 2021.
  4. Matthews KRW, et al. Hastings Cent Rep. 2021;51:47-51.